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The present invention relates to the fabrication of articles from nanoparticulate materials containing isotropically distributed, interconnected pores with cross-sectional diameters in the nanometer and Angstrom range.
Organic and inorganic porous materials play an important role in a number of processing industries such as chemical recovery, purification and dehumidification. Porous ceramic oxides (e.g., clays, silica and zeolites) are used as catalysts or catalyst supports in chemical and petroleum reactions such as hydrocracking, hydrodesulfuration, reforming and polymerization. The high chemical, mechanical and thermal resistance of inorganic membranes makes them attractive in the field of ultrafiltration and absorption drying (e.g. wicking in powder injection molding technology).
As modern technology developments take place at an ever increasing pace, there is a growing demand for materials with controlled microporosity or nanoporosity, to be used in a variety of technologically advanced and highly specialized applications.
For example, attitude control rocket engines used on interplanetary spacecraft such as the Cassini Jupiter space probe, draw argon through flow controllers made from microporous metallic filters.
In the medical field, metallic and ceramic structures with controlled porosity are used as building blocks in tissue engineering, bone grafts, surgical implants, bacterial filters and drug delivery systems.
In the microelectronics industry, as the density of functional elements in integrated circuits increases, the need for improved on-chip interconnections becomes more acute. Such interconnections generally consist of multiple layers of metallic conductor lines embedded in low dielectric constant materials. To reduce capacitive effects leading to cross talk between conductor lines, and to allow for lower voltages to be used to power integrated circuits, materials with low dielectric constants, i.e., typically below 2.5, are desirable as they allow faster signal velocity and shorter cycle times. This has led to the development of dielectric materials with designed-in nanoporosity.
In the field of energy storage and conversion, the ability to tailor the composition, shape and porosity of anodes, cathodes and electrolytes produced from complex metallic, ceramic and cermet compositions, plays an important role in the development and design of low-cost, high efficiency solid oxide fuel cell (SOFC) systems.
In the field of photonics, nano-sized, highly-ordered three-dimensional open porosity structures are potentially useful as photonic bandgaps and optical stop-bands which can be used to fabricate new types of diffractive optical sensors with enhanced sensitivities.
Generally, porous materials are produced in the form of thin layers, films or membranes. The prior art has devised a number of methods for producing such kinds of membranes, e.g. electrochemical etching of alumina or silicon, chemical etching of glasses, ion-track etching of polymers and self-assembly of block copolymers. Methods based on selective etching usually generate straight, one-dimensional channel structures and have been very successful in the manufacture of commercial membrane films. Methods based on self-assembly of block copolymers provide an elegant and efficient route to macroporous films with a regular array of spherical pores. Although these pores are fully opened on the surfaces of the film, they are isolated from each other in the bulk.
In one approach, small hollow glass spheres are introduced into a material. Examples are given in Kamezaki, U.S. Pat. No. 5,458,709 and Yokouchi, U.S. Pat. No. 5,593,526. However the use of small hollow glass spheres is typically limited to inorganic silicon-containing polymers.
In another approach, a thermostable polymer is blended with a thermolabile (thermally decomposable) polymer. The blended mixture is then crosslinked and the thermolabile portion thermolyzed. Examples are set forth in Hedrick et al., U.S. Pat. No. 5,776,990. Alternatively, thermolabile blocks and thermostable blocks alternate in a single block copolymer, or thermostable blocks and thermostable blocks carrying thermolabile portions are mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermolabile blocks. However, many difficulties are encountered utilizing mixtures of thermostable and thermolabile polymers and the distribution and pore size of the nanovoids is difficult to control.
In yet another approach, a polymer is formed from a first solution in the presence of microdroplets of a second solution, where the second solution is essentially immiscible with the first solution. During polymerization, microdroplets are entrapped in the forming polymeric matrix. After polymerization, the microdroplets of the second solution are evaporated by heating the polymer to a temperature above the boiling point of the second solution, thereby leaving nanovoids in the polymer. However, generating nanovoids by evaporation of microdroplets tends to be an incomplete process that may lead to undesired outgassing and potential retention of moisture and employing microdroplets to generate nanovoids often allows little control over pore size and pore distribution.
In yet another prior art method, Schulz, et al., U.S. Pat. No. 6,180,222, generate nanoporous aluminum oxide layers or membranes by anodic oxidation of aluminum anodes in an electrochemical cell in which the electrolyte is usually sulfuric or oxalic acid. Pore diameter is controlled by precisely monitoring the electrolyte composition and temperature as well as the anodizing voltage, the pore diameter increasing as the voltage increases. If a nanoporous membrane is the desired end product, the non-oxidized part of the aluminum anode is machined away or dissolved in an acid bath.
In yet another method of the prior art, Morgart, et al., U.S. Pat. No. 5,242,595 disclose a method of fabricating composite membranes on a porous support structure, usually alpha alumina. In another prior art invention, Boye, et al., U.S. Pat. No. 5,266,207 disclose a technique to fabricate a composite membrane consisting of several layers of oxide of decreasing particle size deposited on a porous tubular support made of carbon or alumina.
Organic nanofiltration membranes present the disadvantage of being mechanically and thermally fragile and sensitive to chemical attack. While porous oxide membranes offer a number of advantages over polymeric membranes such as higher operating temperature ranges, greater structural integrity and improved resistance to corrosion, advanced material compositions are still required for applications under highly specific operational and environmental conditions requiring improved resistance to mechanical impact and thermal shock, water and oxygen resistance, and molecular selectivity to small molecules and gases.
Ceramic materials of the silicon carbide, silicon nitride, silicon aluminide, boron nitride, and related types offer many of the properties needed for advanced applications, however, the sol-gel synthesis methods typically used to prepare porous oxide membranes or catalyst supports are incompatible with the preparation of these because of the need to use water in their preparation. Sintering or reactive sintering of these ceramics likewise produces materials with pore sizes of from about 0.1 to about 1000 microns which are non-uniform and generally too large for effective molecular separation.
Recently, porous silicon carbide and silicon nitride ceramics have been prepared by pyrolysis at temperatures of 1300xc2x0 C. and higher of ceramic precursors, e.g., polycarbosilanes, polysilanes, polycarbosiloxanes, polysilazanes, etc. During pyrolysis, various gaseous decomposition species such as hydrogen and organic compounds are liberated. These gases tend to coalesce as the preceramic precursor crosslinks and hardens, resulting in a lower density ceramic having a macroporous closed-cell structure.
In Dismukes, et al., U.S. Pat. Nos. 5,696,217 and 5,852,088 the synthesis of microporous and nanoporous ceramics with catalytic functionality by ammonia pyrolysis of ceramic precursors is described.
Chemical vapor deposition techniques are also used to produce microporous ceramic layers, but this tends to be an expensive, high temperature process with limited ability to tailor complex ceramic compositions.
In all the above methods, the limitation in the selection of materials, the thinness of the membrane films and the poor control over the size and distribution of pores, are generally viewed as major drawbacks. Also the deposition/etch techniques restrict design complexity while most of the above processes are not readily amenable to mass production.
In accordance with the present invention the problems of the prior art are substantially overcome by providing a method to fabricate complex three-dimensional structures with isotropically distributed, interconnected, controlled nanoporosity. As used herein, the term nanoporosity refers to porosity in which the mean width (diameter) of the pores is in the nanometer or even Angstrom range.
It is a primary object of this invention to overcome the dimensional and design limitations of the prior art in the preparation of porous nanostructures by providing a method to fabricate net-shape porous nanostructures with design features such as deep holes, rounded shapes, angled slots, threaded holes, etc.
It is a second object of this invention to overcome the material limitations of the prior art in the preparation of porous nanostructures by providing a method to fabricate net-shape porous nanostructures from a variety of materials including metals, ceramic, cermets, etc.
It is yet another object of this invention to provide a method to fabricate homogeneous porous nanostructures in which the amount and size of initial or starting porosity can be accurately preset and from there the degree and morphology of the porosity can be precisely achieved.
It is still another object of this invention to provide an economic manufacturing process for porous nanostructures which lends itself easily to automation and mass-production and makes use of well-known prior art techniques such as casting, machining, molding, sintering, etc. It is therefore also an object of this invention is to provide a manufacturing method for porous nanostructures which is easily accessible to the manufacturing industry instead of being restricted to specialized industries, laboratories and academic establishments.